This invention relates to uniformly-foamed thermoplastic polymeric mechanical planarization polishing pads (“MP pads”). More particularly, this invention relates to uniformly-foamed thermoplastic polymeric MP pads made with supercritical fluids.
Multiple layers of conducting, semiconducting, dielectric, and insulating materials are deposited on a substrate during integrated circuit device fabrication. Often, imperfect substrate fabrication and imperfect integrated circuit layer deposition result in formation of undesirable topography (e.g., recesses, protrusions, scratches, etc.) on the substrate and on one or more of the deposited layers. Because undesirable topography can compromise the integrity of an integrated circuit device (e.g., a topographical recess in a dielectric layer can impose step coverage problems with the deposition of another integrated circuit layer, undesirable topology can cause depth of focus issues during photolithography, etc.), the substrate and each deposited layer of an integrated circuit device are preferably planarized (i.e., made level) before additional layers of integrated circuit material are deposited.
Known mechanical planarization (“MP”) processes are used to remove undesirable topology from layers of integrated circuit material. Generally, an MP pad rotating about a line preferably perpendicular to the surface of an integrated circuit wafer is brought into contact with that surface during an MP process. The rotating MP pad mechanically polishes (i.e., removes undesirable topography from) the surface material of the integrated circuit wafer. Concurrently, a fluid-based chemical (i.e., a chemical polishing “slurry”) that reacts with the integrated circuit material (i.e., for a chemical-mechanical planarization (“CMP”) process) or an inert liquid applied to the MP pad facilitates the removal of undesirable topography. For example, an inert liquid applied to the interface between an MP pad and an integrated circuit wafer can facilitate the removal of mechanically-ground integrated circuit material.
The porosity of an MP pad is often controlled to positively affect the material removal rate of an MP process. In particular, the porosity level of an MP pad directly influences and can increase the material removal rate, because the “pores” of an MP pad retain and distribute chemical or inert polishing liquid that facilitates the planarization of undesirable topography. However, a significantly porous MP pad may be undesirable unless the MP pad pores are both uniform in size and uniform in distribution throughout the MP pad. Because uniform MP pad pores evenly distribute polishing liquid to the surface of an integrated circuit wafer, a uniformly porous MP pad contributes to a desirable uniform material removal rate across the surface of the integrated circuit wafer. Thus, uniformity in the porosity level of the MP pad (i.e., uniformity in porosity level across the surface and throughout the bulk of a single MP pad and uniformity in porosity level from MP pad to pad) is an important MP pad characteristic.
Various known fabrication methods produce porous MP pads. For example, the known method of including hollow microbeads in a liquid prepolymer imparts porosity in thermoset polymer MP pads (e.g., Rodel IC1000 MP pad). As another example, the known method of coating a porous network of felt or woven fibers with a thermoset polymer also imparts porosity in thermoset polymer MP pads (e.g., Thomas West 711 MP pad). In another known method, perforations (i.e., slurry “cups”) or through-holes are cut or molded into a polymer to provide porosity in MP pads. In still another known method, direct foaming of thermoset polymers using a non-supercritical fluid foaming agent produces porous thermoset polymer MP pads (e.g., Universal Photonics ESM-U MP pad).
Thermoset polymer MP pads may be, however, problematic. In particular, because thermoset polymers are generally formed in thick “cakes” that are characteristically non-uniform over the surface and throughout the bulk of the cake (which is caused by a non-uniform temperature of the cake during curing of the thermoset polymer), individual thermoset polymer MP pads mechanically skived (i.e., cut) from a thermoset cake are likely to exhibit unpredictable irregularities and non-uniformity. Further, mechanically skiving a thermoset cake can introduce surface and bulk irregularities such as, for example, fracturing and abrasions in thermoset MP pads. Thus, thermoset polymer MP pads are often characterized by undesirable non-uniformity across the surface and throughout the bulk of a single MP pad and by undesirable non-uniformity from MP pad to pad.
Because non-uniform MP pads can produce undesirable non-uniformity in the surface of an integrated circuit wafer during polish of that wafer, it may not be desirable to use caked thermoset polymer MP pads in an MP process. In addition, because mechanically cutting thermoset cakes to produce thermoset MP pads typically results in significant material waste (i.e., the unusable material cut from the edges of thermoset cakes), methods of fabricating thermoset MP pads from caked thermoset polymers may not be cost-effective. Note that although single thermoset polymer MP pads may be formed via reaction injection molding (“RIM”), difficulty in controlling the ratio of components of the thermoset polymer during injection and in controlling the temperature of the thermoset polymer during thermoset polymer curing causes these pads to be especially non-uniform.
In contrast to thermoset polymers that are generally formed in thick cakes, thermoplastic polymers are generally formed (e.g., molded or extruded) in single sheets or units at a time. Thus, thermoplastic polymeric MP pads can be advantageously individually fabricated and generally do not require mechanical skiving that can cause MP pad defects and material waste.
For example, Cook et al. U.S. Pat. No. 6,325,703 describes a method of fabricating porous thermoplastic polyurethane MP pads by sintering. In particular, dry thermoplastic polyurethane resins are placed in an individual MP mold and “welded” together via a heating cycle (at temperatures below the melting point) to produce a porous thermoplastic polyurethane MP pad. However, sintered thermoplastic polymeric MP pads may be problematic. In particular, because dry thermoplastic resins are often imperfectly mechanically ground to a predetermined size before they are sintered, and because slight variations in resin size can result in undesirably non-uniform pores, sintered thermoplastic polymeric MP pads can be undesirably non-uniformly porous. Further, uneven pressure and uneven distribution of dry thermoplastic resins in an MP pad mold can result in sintered thermoplastic polymeric MP pads that are non-uniformly porous.
As another example, Budinger et al. U.S. Pat. No. 4,927,432 describes a method of coalescing a solubilized thermoplastic polymer with a porous network of felt or woven fiber to impart porosity in thermoplastic polymer pads. However, because the thermoplastic pad derives its porosity from the projecting ends of the porous network, and because these projecting ends are somewhat randomly distributed, thermoplastic MP pads made by coalescing thermoplastic polymer with felt or woven fiber are often non-uniformly porous.
Other products (e.g., polystyrene packaging, high density polyethylene bottles, etc.) use known fabrication methods to produce porous thermoplastic polymeric materials that are both significantly porous and uniform in porosity. In particular, known fabrication methods using supercritical fluids produce foamed thermoplastic polymeric materials (which are characteristically porous) that are both significantly porous and uniform in porosity. For example, methods of fabricating foamed thermoplastic polymeric materials using supercritical fluids are described in Cha et al. U.S. Pat. No. 5,158,986, Park et al. U.S. Pat. No. 5,866,053, Blizard et al. U.S. Pat. No. 6,231,942, Park et al U.S. Pat. No. 6,051,174, and Blizard et al. U.S. Pat. No. 6,169,122. In the known methods, a rapid change in the solubility and volume of a supercritical fluid dissolved in a thermoplastic polymer results in foaming of the thermoplastic polymer. Moreover, because thermoplastic polymeric scrap material can be reprocessed, these known methods of fabricating foamed thermoplastic polymers using supercritical fluids can eliminate process waste of thermoplastic polymeric material. However, such methods are not known for fabricating MP pads.
In view of the foregoing, it would be desirable to use known methods of fabricating foamed thermoplastic polymeric materials using supercritical fluids to fabricate foamed thermoplastic polymeric MP pads.